Abstract. Critical ventilation velocity remains the most studied phenomenon in the tunnel fire literature. But focus on the velocity as the principal parameter may hide other features of the interaction between a fire and tunnel flow. The throttling effect was identified in the 1960s and described in the 1970s, yet it seems to be generally overlooked these days. In essence, the throttling effect is the tendency of a fire in a tunnel to resist the airflow; the larger the fire, the greater the resistance. Thus, while it has been shown that no increase in longitudinal flow velocity is required to control smoke from fires larger than the 'super critical' limit, in practice, an increasing number of ventilation devices are required to achieve this flow, and hence control the smoke from a fire, as the fire size grows. A CFD modelling study using the FDS model has been carried out to demonstrate this effect. It is clearly demonstrated that for fires larger than the 'super critical' limit, an increasing number of jet fans are required to control the smoke from increasingly larger fires. The results of the modelling study are presented.